Jekyll new theme redesign

This is a work in progress, but nearly complete. Opening PR early (GitHub style) for discussion and finally polish.

I did all of my work so far on a separate repo ([jglovier/jekyll-new](https://github.com/jglovier/jekyll-new)) to avoid cluttering the commit log with things I was exploring, then re-performed the work on the files within jekyll/lib/site_template for a more streamline commit log. 

You can preview the intended design work via the live gh-pages branch from jglovier/jekyll-new at [jekyll-new.joelglovier.com](http://jekyll-new.joelglovier.com/). I'm hoping I didn't miss anything in transfering the files over to my actual Jekyll repo, but for some reason I wasn't able to run `jekyll serve` locally on the version within lib/site_template to double check.

The work is mostly complete, although there are a few things I need to do before the PR is really ready to merge, including:

- [x] add mobile styling for nav (thinking of a simple jQuery or CSS trick to convert to hamburger menu at mobile viewport)
- [x] replace `github.com` and `twitter.com` in footer with inline svg logo icons to conserve horizontal space
- [x] take quick pass back over everything in Browserswarm and on mobile to check for layout/design bugs
- [x] ~~possibly address the syntax highlighting (which I didn't touch)~~ *(move to another PR)*
- [x] ~~convert CSS to Sass which is available in Jekyll 2.0 alpha~~ *(move to another PR)*